US3916079A

US3916079A - Coolant feed for high voltage apparatus

Info

Publication number: US3916079A
Application number: US476456A
Authority: US
Inventors: Hubert Kohler; Fritz Schmidt
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1973-06-22
Filing date: 1974-06-05
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28
Also published as: CA1003235A; FR2241881A1; DE2331869B2; JPS5038491A; DE2331869A1; CH573162A5; FR2241881B1; GB1425537A; JPS5419156B2

Abstract

A coolant feed for electrical apparatus having conductors at a high-voltage potential in which the coolant feed line is surrounded by a vacuum vessel in which is located a radiation shield of electrically insulating material containing a plurality of tubes arranged concentrically to the feed pipe through which a cooling medium flows, the tubes forming at least one outgoing line for the coolant in the direction of the axis of the feed pipe and at least one return line in the opposite direction thereby providing a coolant feed with low internal thermal losses and which is well suited for electrical apparatus having high conductor potentials.

Description

United States Patent Kohler et al.

COOLANT FEED FOR HIGH VOLTAGE APPARATUS Inventors: Hubert Kohler, Eltersdorf; Fritz Schmidt, Erlangen, both of Germany Siemens Aktiengesellschaft, Munich, Germany Filed: June 5, 1974 Appl. No.: 476,456

Assignee:

Foreign Application Priority Data June 22, 1973 Germany 2331869 U.S. Cl. 174/15 BH; 174/15 C; l74/DIG. 6 Int. Cl. H01B 7/34 Field of Search 174/15 C, 15 BH, 15 R,

l74/DIG. 6; 62/338, 339, 440, 451, 443

References Cited UNITED STATES PATENTS 12/1964 Silver 174/DIG. 6

Primary Examiner-Arthur T. Grimley Attorney, Agent, or FirmKenyon & Kenyon Reilly Carr & Chapin 57 ABSTRACT A coolant feed for electrical apparatus having conductors at a high-voltage potential in which the coolant feed line is surrounded by a vacuum vessel in which is located a radiation shield of electrically insulating material containing a plurality of tubes arranged concentrically to the feed pipe through which a cooling medium flows, the tubes forming at least one outgoing line for the coolant in the direction of the axis of the feed pipe and at least one return line in the opposite direction thereby providing a coolant feed with low internal thermal losses and which is well suited for electrical apparatus having high conductor potentials.

10 Claims, 1 Drawing Figure COOLANT FEED FOR HIGH VOLTAGE APPARATUS BACKGROUND OF THE INVENTION This invention relates to coolant feedsfor high voltage conductors in general, and more particularly, to an improved coolant feed with low thermal losses.

In electrical apparatus having conductors cooled to a low temperature which conductors are at a high potential, a cryogenic medium must be fed-in from the outside in order to cool the conductors. Arrangements of this nature are used, for example, with various types of superconductors, such as superconducting cables, coils and machines. In such operation, the superconductors must always be maintained at'a temperature below their transition temperature. To do so requires a continuous supply of cryogenic medium. It has been discovered that the only practical cryogenic medium for cooling superconductors is helium. Thus, typically, helium is fed-in from the outside but must first be brought to the high voltage potential of the conductor before it is allowed to 'come in direct contact therewith.

In operating a feeding-in or discharge device for-a cryogenic medium, in particular for helium, provisions must be made to insure good thermal insulation along with the necessary dielectric strength under all operating conditions. That is to say, that the line leading from the cryogenic supply to the high voltage conductor to be cooled must be well insulated to insure that the coolant is effective upon reaching the conductor and also must be constructed with the necessary dielectric strength because of the large potential difference between one end and thev other of the feeding arrangement. These requirements lead to certain difficulties. Thermally well insulated lines are normally located in a high vacuum and are generally surrounded by metallic highly reflecting surfaces and, in some cases, metallized plastic foils also referred to as superinsulation. However, such metallic mirror-coating and metallized plastic foils-are not usable in a coolant feed apparatus at high voltage potential since they couldlead to short circuits of the high voltage. Mirror-coatings using semiconductor materials generally are also not sufficient since the highest reflectivity which can be obtained from such materials is only 40 to 45 percent for example, in the case of germanium or silicon One proposed solution to the problem is disclosed in U.S. Pat. No. 3,522,361 in which arrangement several normal conductors are led throughseveral cooling chambers each representing a cooling stage between room temperature and the superconductor temperature. The cooling stage with the lowest temperature level is cooled by liquid helium. Therein, the normal conductors are joined to superconductors of a cable. The conductors which are designed for a high voltage potential are disposed between the individual cooling stages in insulator bodies which are required to prevent flash-over between the outer parts of the feed arrangement at ground potential and the conductors. The helium bath of the last cooling stage which also is used for cooling the superconductors in the connected cable is replenished through a pipe. This pipe is concentrically enclosed by a larger pipe through which evaporating helium from the bath or the cable can escape-The pipes are made of insulating material. Because of the relatively short length'of the feed line for the coolants, particularly the one for the liquid helium, the current and coolant arrangement has a dielectric strength which is quite low, i.e., that of helium. As a result, the operating voltage of the connected cable must then be chosen at a relatively low value. Furthermore, this arrangement has rather high coolant losses.

Thus, it can be seen that there is a need for an improved coolant feed arrangement which is suitable for electrical apparatus having high conductor potentials with the conductors maintained at cryogenic temperatures and which feed arrangement has low internal thermal losses and meets the other requirements noted above.

SUMMARY OF THE INVENTION The present invention solves this problem by placing in a vacuum vessel which has a cylinder length in the axial direction of the feed pipe determined by the dielectric strength ofits outer cylinder surface a radiation shield of electrically insulating material through which a cooling medium flows in several tubes arranged concentrically to the feed pipe and to the cylinder surface of the vacuum vessel. These tubes form at least one outgoing line for the cooling medium in the direction of the axis of the feed pipe and at least one return line in the opposite direction.

This arrangement achieves particular advantages. Complicated arrangements for generating a breakdown-proof potential gradient in the electrical apparatus itself are not necessary since the coolant is fed to the electrical apparatus at the high voltage potential of the conductors. The dielectric high voltage strength of the feeding arrangement is insured by a sufficient length of the feed pipe. Thus, in accordance with the preferred embodiment of the invention, the coolant feedpipe has a length corresponding at least to the length of the vacuum vessel which is designed in order to obtain the required breakdown resistance at its outer cylinder surface. Since this breakdown strength is always lower than that of the coolants in the feed pipe, adequate dielectric strength of the cooling carrying section is then always assured.

BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE is a longitudinal cross section in schematic form illustrating a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The FIGURE illustrates an arrangement for feeding a coolant such as helium into an electrical apparatus such as superconducting cables, coils or machines in which the conductors are at high potential. This coolant designated A and which is preferably helium is contained in the liquid state in a cooling tank 2. Connected to the cooling tank through a pressure reducing valve 4 is a pressure bottle 3 containing gaseous helium. The pressure of the gaseous helium on the surface of the liquid helium in the tank 2 causes the liquid helium A to flow from the coolant tank through a connecting pipe 5, a corrogated pipe section 7 and then through a feed pipe 8 with an end piece 9 to an electrical apparatus not shown on the FIGURE.

The feed pipe 8 is made of an insulating material. Within it the helium A traverses a potential gradient starting at ground, i.e., the tank 2 will be at ground potential, until it reaches a high voltage potential at the end piece 9. As a result, the conveyed helium A can then be fed through this end piece directly to the electrical apparatus without further need for devices to establish a breakdown proof potential gradient. The feed pipe 8 along with its end piece 9 and corrugated pipe section 7 are enclosed within an outer pipe 10. This outer-pipe 10 is the vacuum vessel. Pipe or vacuum vessel 10 is evacuated in conventional fashion through a A 1 l8 and 1 l9. vessel 10 is a radiation shield 12 concentric therewith.

The radiation shield 12 encloses the feed pipe over its entire length. In the illustrated embodiment, the radiation shield comprises a triple tube with two annular cross sectional areas surrounding each other concentrically. It is made, of an electrically insulating material and contains an inner zone 13 bounded by an inner tube 14 and a middle tube 15 representing one flow space for a cooling medium B. Another flow space 17 ,is situated between vthe middle tube 15 and the outer tube 18. As illustrated, a second cryogenic medium B is fed into the inner zone or flow space 13 through a tube connection 19 at the lower end which is at ground potential. The medium E rises in the flow space 13 and is conducted at the upper high voltage end 16 into the outer flow space 17 returning in the latter to the bottom. At the bottom, it leaves through a connection 20 at the same height as connection 19. The cryogenic medium E which flows through the radiation shield 12 may be liquid nitrogen at about 77K.

By causing this flow therethrough, the shield prevents heat conduction or radiation from the outside to the heliurncarrying feed pipe 8. The medium B fed into the radiation shield 12 at connection 19 which is at ground potential, during its rise, traverses a potential gradient within the flow space 13 up to the high voltage potential at the end 16 and again traverses this gradient downward until it is again at ground potential when it flows back out through the outlet connection 20.

The feed pipe 8, the radiation sield 12 in the form of a triple tube and the vacuum vessel 10 enclosing these, can be made of glass, quartz or also ceramics. A particularly suitable material is molybdenum lead glass. Also suitable are plastic materials which give off little gas in a high vacuum. In order to securely position the feed pipe 8, the radiation shield 12 and the outertube 10 with respect to each other, support members 21 are provided. These may be in the form of projections and may be made of one of the above-mentioned insulating materials. Through this arrangement, the vacuum vessel 10 which is at normal temperature is thermally insulated by the support members 21 from the radiation shield 12 which is at the temperature of the cryogenic medium B and from the feed pipe 8 carrying the coolant A. As a result, frosting of the vacuum vessel 10 and an attendent reduction of its dielectric strength is prevented.

In the feed arrangement of the present invention, provision is-made that the feed pipe 8 is sufficiently long to insure an adequate dielectric strength of the coolant section. Such will always be the case if the 5 length of the feed pipe 8 approximately corresponds to the length of the vacuum vessel 10. As noted above, the length of the vacuum vessel 10 is determined by the dielectric strength of its outer cylinder surface in air. Since the dielectric strength of the feed pipe 8 carrying 10 the coolant A and of the radiation shield 12 carrying the coolant B is always better in a vacuum than in air [the dielectric strength of liquid nitrogen, for example being about 30 percent better than that of liquid helium and considerably better than that of gaseous helium] 5 this relationship between the length of the feed pipe and the length of the vacuum vessel will always insure adequate dielectric strength. Thus, the length of the vacuum vessel 10 and thus that of the radiation shield 12 and feed pipe 8 is determined by the dielectric 20 strength of the outer cylinder surface of the vacuum vessel 10 in air. It will be recognized that the different shrinkage of the material of the tubes, particularly that of the radiation shield 12 with respect to the feed pipe 8 and the vacuum vessel 10 must be taken into consid- 25 eration. In order to make provisions for such effects,

30 It is advantageous if corona shields are provided at both ends of the arrangement. On the FIGURE, a single high voltage potential corona shield 23 is shown at the top of the arrangement as an example.

It is also advantageous to provide a separate helium pump for pumping the liquid helium A from the coolant tank to the feed pipe 8 through the connecting pipe 5 and corrogated pipe 7. Such a pump 24 is shown on the FIGURE and as illustrated, can be placed in the coolant tank 2. In some cases, a pump 24 will be capable of generating sufficient pressure to transport the helium A at the required flow rate in which case the pressure bottle 3 and pressure reducer 4 can be eliminated.

With the illustrated arrangement, not only helium which is a preferred cryogenic medium, but any cyrogenic medium can be supplied to electrical apparatus at high voltage potential. Not only is the medium conducted to the electrical apparatus in a thermally and electrically insulated manner but the apparatus at the same time bring the medium from ground potential to the high voltage potential of the electric apparatus. When feeding and discharging helium, a number of operating states are possible as follows: Boiling helium, liquid undercooled helium at a pressure higher than 1 atm, supercritical helium at a pressure higher than 2.3 atm, and also gaseous helium in the temperature range of up to about 10 K. Of these operating states, gaseous helium has the lowest dielectric strength as is well known in the art. The danger of a reduction of the dielectric strength becomes particularly great if the transfer of the coolant is carried out in the illustrated feed arrangement in an intermittent manner rather than continuously. Such may occur, for example during the replenishment of ing operations so that, upon resumption of the pumping of the boiling helium, a gas pulse is first pushed through the feed arrangement, the gas pulse being made of relatively warm helium gas. As is well known, the dielectric strength of gaseous helium is reduced. considerably with increasing temperature. This possibility leads to the requirement of good thermal insulation as well as the necessity for good dielectric strength under all operating conditions during feeding.

As long as the helium A flows continuously through the feed arrangement in any of the above mentioned states, no difficulties can occur since the heat radiated into the helium carrying feed pipe from the radiation shield 12 filled with liquid nitrogen is very small. As a result, the temperature of the liquid or gaseous helium is increased only to the slighest possible extent because of the support elements 21 which have low heat conductivity and are disposed at a large distance from each other. However, even for the above mentioned case where a gas pulse is pushed through the feed with the pulse made up of relatively warm helium gas, a helium carrying pipe which has a length equal to that of the outer air distance of the vacuum vessel providesa sufficient margin of safety. During intermittent operation the normally helium carrying feed pipe can at most warm up to the temperature of the nitrogen because of the surrounding triple tube of the radiation shield 12 which is nitrogen cooled and is always kept filled. At this temperature, the dielectric strength of the helium is still about 2.5 times that of helium at room temperature. To further take into account problems of this nature, it is also advantageous to increase the dielectric strength by dividing the feed pipe 8 into individual capillaries connected in parallel for the flow of helium gas or liquid.

It should be noted that in principle, it is also possible to use liquid hydrogen rather than liquid nitrogen for thermal shielding. Since approximately the same breakdown values exist for hydrogen as for nitrogen, this substitution results in no limitations with regard to the inwardly directed radiated heat and heat conduction.

Thus, an improved coolant feed for high voltage conductors in an electrical apparatus has been shown. Although a specific embodiment has been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit of the invention which is intended to be limited solely by the appended claims.

What is claimed is:

l. A coolant feed arrangement for electrical apparatus having conductors which are to be cooled to a low temperature and which are at a high potential comprismg:

a. a feed pipe made of an electrically insulating material for conducting a first coolant from a coolant source to the electrical apparatus;

b. a vacuum vessel made of insulating material, having a cylinder length in the direction of the axis of the feed pipe determined by the dielectric strength of its outer cylinder surface, surrounding said feed pipe concentrically; and

c. a radiation shield made of electrically insulating material and including a plurality of concentric tubes through which a second coolant can flow, said radiation shield being arranged concentric to said feed pipe and said vacuum vesseL-and having at least one inlet line and one return line for coupling said second coolant into and out of said direction.

2. A coolant feed according to claim 1 and further including a first coolant and means to supply said first coolant and wherein said first coolant supplied is helium.

3. A coolant feed according to claim 1 and further including a second coolant and means supplying said second coolant and wherein said second coolant for said radiation shield is a cryogenic medium having a temperature higher than the temperature of said first coolant.

4. A coolant feed according to claim 1 wherein said feed pipe, radiation shield and vacuum vessel are made of one of the group consisting of glass, quartz and ceramics.

5. A coolant feed according to claim 1 wherein the material of which said feed pipe, radiation shield and vacuum vessel are made, is a plastic material which gives off little gas in a high vacuum.

6. A coolant feed according to claim 1 wherein said radiation shield has a high voltage side and a side at ground potential and wherein said radiation shield comprises a. an inner tube;

b. a middle tube; and

c. an outer tube to thereby form inner and outer flow passages between said tubes with the inner flow passage connected to said inlet line, the outer flow passage connected to said outlet line and said inner and outer passage connected with each other at the high voltage side of said radiation shield.

7. A coolant feed according to claim 6 wherein helium is the coolant.

8. A coolant feed according to claim 7 wherein the second cooling medium for said radiation shield is a cryogenic medium having a temperature higher than the temperature of said first coolant.

9. A coolant feed according to claim 8 wherein said feed pipe, radiation shield and vacuum vessel are made of one of the group consisting of glass, quartz and ceramics.

10. A coolant feed according to claim 8 wherein said feed pipe, radiation shield and vacuum vessel are made of molybdenum lead glass.

Claims

1. A coolant feed arrangement for electrical apparatus having conductors which are to be cooled to a low temperature and which are at a high potential comprising: a. a feed pipe made of an electrically insulating material for conducting a first coolant from a coolant source to the electrical apparatus; b. a vacuum vessel made of insulating material, having a cylinder length in the direction of the axis of the feed pipe determined by the dielectric strength of its outer cylinder surface, surrounding said feed pipe concentrically; and c. a radiation shield made of electrically insulating material and including a plurality of concentric tubes through which a second coolant can flow, said radiation shield being arranged concentric to said feed pipe and said vacuum vessel, and having at least one inlet line and one return line for coupling said second coolant into and out of said direction.

6. A coolant feed according to claim 1 wherein said radiation shield has a high voltage side and a side at ground potential and wherein said radiation shield comprises a. an inner tube; b. a middle tube; and c. an outer tube to thereby form inner and outer flow passages between said tubes with the inner flow passage connected to said inlet line, the outer flow passage connected to said outlet line and said inner and outer passage connected with each other at the high voltage side of said radiation shield.

7. A coolant feed according to claim 6 wherein helium is the coolant.